An apparatus of the general type of that to which the present invention relates has been described in French Pat. No. 957,393. In this apparatus, the compartments are formed by small-diameter ducts with walls which are not pervious to liquid, these ducts being permanently filled with liquid which is pressure-fed counter-currently; the drum is thus substantially completely filled with liquid and each duct forms a small individual extractor inside which the solids are permanently contacted with the liquid.
Such known apparatus has for a drawback the high risk of duct packing and this is the reason why additional means for moving the solids, such as conveyor screws, are mounted inside the ducts. Moreover the apparatus does not allow mixing of the solids particles and the liquid, which function is required to insure a good extraction of some materials such as when extracting sugar from beet or cane cossettes.
For extracting sugar from beet cossettes, there are known, notably from Belgian Pat. Nos. 475,626 and 711,219, so-called "diffuser" apparatus which comprise communicating compartments formed between the windings of a plurality of conveyor screws and walls pervious to the liquid; the walls are used for raising solids which are dipped in sequence in the liquid, separated therefrom and moved along the axial direction from one compartment to another.
Such known diffusers insure a thorough extraction notably because the liquid is divided into portions having increasing concentration rates which do not meet one another. However, the drawback thereof lies in the construction complexity thereof.